Resin transfer molding process

ABSTRACT

The present invention relates to a seal pin indexed precisely in a resin transfer molding die to assure the desired resin-to-fiber ratio in the mold part, and, thereby, to produce reliably and consistently a part of minimum weight with adequate strength. The index/resin seal pin of the present invention for RTM mold dies is novel in that it provides the seal (via replaceable O-rings) on the threaded stem of the pin to prevent low viscosity resin from invading the threads, bolts pins, or other elements of the internal mandrel. The pin allows the entire inner mandrel to be enclosed and sealed within the mold cavity, simplifying the sealing system. The invention also relates to a method for using the seal pin in a resin transfer molding process.

REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. Provisional PatentApplication 60/128,463, filed Apr. 9, 1999.

TECHNICAL FIELD

The present invention relates to a seal pin indexed precisely in a resintransfer molding die to assure the desired resin-to-fiber ratio in themold part, and, thereby, to produce reliably and consistently a part ofminimum weight with adequate strength.

BACKGROUND ART

Reducing the cost of aerospace parts requires a switch to reliable,repeatable processes that yield parts, which meet the rigorousrequirements of aerospace. In aerospace, the issues of absolute weight,shape, strength, and durability must be traded during the design of apart. A factor often ignored during this design trade is the cost ofmanufacture. Often, however, safety forces part designs that areexpensive to manufacture. Resin transfer molding (RTM) is promising as aprocess to replace complicated built-up metal assemblies with one-piecefiber-reinforced resin composite alternatives. One challenge with RTM isensuring that the resin-to-fiber ratio in the molded part is adequateeverywhere in the part so that the molded part will have the designstrength and durability. The present invention is an improvement in RTMusing a seal pin in the mold die to index the preform for the partreliably and precisely on the mold cavity on an inner mandrel that iscompletely sealed within the die set.

Resin transfer molding (RTM) is a process by which a resin is pumped atlow viscosities and low pressures into a closed mold die set containinga preform of dry fabric to infuse resin into the preform and to make afiber-reinforced composite part. The RTM process can be used to produceat low cost composite parts that are complex in shape. These partstypically require continuous fiber reinforcement along with inside moldline and outside mold line controlled surfaces. The ability to includeand place continuous fiber reinforcement in large and small structuressets RTM apart from other liquid molding processes.

Some RTM mold die sets require outer dies to define a cavity and aninner mandrel that seats in the cavity to define a hollow region in themolded part. Often the mandrel must be assembled from multiple parts sothat it can be disassembled and removed from the molded part. RTM diesets that used mandrels usually are configured so that the mandrel isinserted through an opening in the dies. Sealing the base plate of themandrel with the opening in the dies was challenging. With theseal/index pins of the present invention, the mandrel can be positionedentirely within the mold cavity, allowing a simpler seal system betweenthe dies.

SUMMARY OF THE INVENTION

The present invention relates to a seal pin indexed precisely in a resintransfer molding die to assure the desired resin-to-fiber ratio in themold part, and, thereby, to produce reliably and consistently a part ofminimum weight with adequate strength. The index/resin seal pin of thepresent invention for RTM mold dies provides a seal (via replaceableO-rings) on the threaded stem of the pin to prevent low viscosity resinfrom invading the threads, bolts pins, or other elements of theassembled parts that form the inner mandrel. The pin allows the entireinner mandrel to be enclosed completely and sealed within the moldcavity, simplfying the sealing system. Preventing resin flow into themandrel's parts allows their easy disassembly following injection of theresin and molding of the part without causing damage to the part. Thepin seats in a complementary recess in the dies to provide closetolerance indexing of the mandrel in the mold cavity. Without thisindexing, the mandrel would not position the fiber preform accuratelywithin the mold cavity. Following injection of the resin and cure, themolded part could have areas starved of resin or areas too rich inresin. Starved areas would have insufficient strength while rich areasintroduce unnecessary weight.

The RTM process initially evacuates the mold cavity, exposing themandrel to a high vacuum. Resin injection follows with an injection andcure pressure of up to 300 psi. Therefore, proper sealing is a criticalfeature for obtaining quality parts.

The present invention relates to the seal/index pin for the innermandrel, to dies for accommodating the pin-mandrel assembly, and to RTMmethods for using the seal/index pins on a mandrel completely enclosedwithin the mold cavity to mold composite parts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an assembly drawing of a preferred resin transfer molding(RTM) die using a seal (or index) pin of the present invention.

FIG. 2 is an isometric showing a preferred seal pin.

FIG. 3 is an elevation of a seal pin of the type shown in FIG. 2.

DETAILED DESCRIPTION

The seal/index pin of the present invention and its use in an improvedRTM process for reliably and reproducibly molding aerospace parts ofadequate strength will be described with reference to molding a drainmast. A preferred die set is shown in FIG. 1 where a matched dies 10 and12 are fit together around a drain mast mandrel 14. Seal/index pins onthe mandrel seat in complementary recesses in the dies to position themandrel accurately in the mold cavity cut into the dies. In theparticular die set shown in FIG. 1, we are able to mold two drain mastssimultaneously. The bottom die 10 includes forming impressions orrecesses 16 cut into the die material (typically P20 steel) to definethe outer mold line (OML) on one surface of the drain mast. A channel 18for injection of resin to the impressions 16 is also cut into die 10. Ableed channel 19 ensures that the die cavity defined by the matchedimpressions in dies 10 and 12 is filled completely during the injectionof resin. This bleed channel directs overflow of resin from the moldcavity out of the die set. One or more (in FIG. 1, two) peripheralchannels 20 in die 10 form seats for seal rings that allow an evacuatedor pressurized environment around the mold impressions 16 when the dies10 and 12 are closed and sealed. The upper die 12 also includes moldimpressions 26 corresponding to the OML of the drain masts. Index posts22 register with corresponding seats 24 in the upper die 12 foraccurately positioning the recesses or impressions in the dies to definethe OML of the drain mast in a mold cavity.

When the dies 10 and 12 are clamped together, the correspondingimpressions 16 and 26 define the mold cavity that corresponds with theOML of the drain mast, in this case. The drain mast is a hollow part. Tomold it, an inner mandrel 28 must be used to define the hollow bydefining the inner mold line. The index/seal pins 30 of the presentinvention allow the mandrel to be sealed completely within a mold cavityformed into the dies 10 and 12. A fiber preform is laid up on themandrel 28, and positioned in the mold cavity before the dies 10 and 12are closed. After molding and cure of the injected resin, the moldedpart is removed from the cavity and the mandrel 28 is removed within thepart. Our goal is to achieve the proper ratio of resin to fiber in allareas of the part. Doing so will produce a molded part having adequatestrength. To achieve this part, the mandrel (which defines the innermold line of the drain mast) must be accurately positioned in the dieset to close tolerance to position the fiber preform accurately. Weaccomplish the accurate positioning with the seal/index pins 30. Apreferred pin 30 is shown in isometric in FIG. 2 and in detailedelevation in FIG. 3.

For making a drain mast, three seal/index pins 30 are used for eachpart. Two pins are seated in threaded bores on a face of the mandrel,which defines the mouth and mounting flange of the drain mast. Each pin30 includes a threaded stem 32 that screws into the mandrel. A sealingO-ring 34 assures a resin seal between the mandrel and the pin. Wheninstalled, the pin 30 projects from the mandrel 14 with a bulbous head36 that has a smooth surface complementary with a seat 38 recessed intothe dies 10 and 12 to prohibit migration of resin around the pin duringthe injection step. Leakage of resin will cause the dies to sticktogether and forces considerable repair time to prepare the dies foranother manufacturing run. We have had success with sealing the diesusing the smoothly curving shape shown in FIGS. 2 and 3, but othershapes for the head undoubtedly can be used. Our preferred shape selfcenters in the pin seats 38 when the mandrel is loaded into the dies,which helps us to achieve the desired tolerance and accurate positioningof the fiber preform. For aerospace parts where the goal is to produce apart of minimum weight with minimum adequate strength, the tolerancesare tightly controlled.

For the drain mast, the third pin 30 seats at the end of the spout ofthe part and seats in an extension of the molding impression in thedies. To reduce part count, all the pins are the same. The design inthis case is driven by the size of the spout tube. Commonality is adesirable feature, but is unnecessary.

If indexing is off, the fiber resin ration will cause inconsistency inthe part being fabricated. Inconsistency results in the part beingscrapped because some areas are resin rich while others suffer resinstarvation. Such a part has inadequate strength.

When molding a drain mast, we prefer to use PR500 epoxy, a 350° F. cureepoxy available from 3M. Our process, however, is adaptable to any resintransfer molding resin, although die design may be needed to handleresins having a high percentage of volatiles. During injection of PR500,we apply a pressure of about 40 psi. Once the die is filled, we apply astatic head of about 200-300 psi (preferably 300 psi) to assure that thepart will be free of porosity. Injection and cure for PR500 occurs at350° F. Other resins may require different temperatures. The drain mastis reinforced with fiberglass cloth, but the process is amenable tocarbon fiber or other reinforcement in woven, unidirectional tape, or3-D stitched configurations.

Controlling the positioning of the mandrel in the dies with theindex/seal pins ensures molding of a part having the desiredresin-to-fiber ratio at every feature in the part. Such molding alsoensures a part having minimum weight and the desired strength.

While we have described preferred embodiments, those skilled in the artwill recognize alternatives, modifications, or variations, which mightbe made without departing from the inventive concept. The examplesillustrate the invention and are not intended to limit it. Therefore,the description and claims should be interpreted liberally with onlysuch limitation as is necessary in view of the pertinent prior art.

We claim:
 1. A method for assuring adequate strength throughout a moldedpart made by resin transfer molding, comprising the steps of: (a)applying a fiber preform around a mandrel; (b) positioning the mandrelprecisely in a mold using sealing index pins that assure closetolerances between the fiber preform and the outer mold line of the partas defined by the mold, the sealing index pins including aself-centering bulge adapted for seating in a positioning and indexinggroove in the mold; (c) injecting resin into the mold so that thesealing index pins prevent resin from bleeding out of the areasurrounding the preform and maintain the close tolerances to assure amolded part of desired fiber-resin ratio throughout and adequatestrength.